Here's the CDC (yeah I know, not always a reliable source, but the Chinese government guide seems to back it up):

The virus is thought to spread mainly from person-to-person.

  • Between people who are in close contact with one another (within about 6 feet).
  • Through respiratory droplets produced when an infected person coughs or sneezes.

These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs.


It may be possible that a person can get COVID-19 by touching a surface or object that has the virus on it and then touching their own mouth, nose, or possibly their eyes, but this is not thought to be the main way the virus spreads.

Wait, what?? This is wildly at odds with the anti-transmission messaging I've heard. Where I live (USA), I hear endless appeals to wash hands, not shake hands, and not touch your face, etc. I hear barely a whisper about the grave risks of being indoors in a public, poorly-ventilated space. I mean, that CDC page seems to imply that standing in a poorly-ventilated grocery store, even >6 feet from others, may well be riskier than touching a point-of-sale touchscreen and then immediately touching your face. (Remember, the virus stays in the air 30 minutes. [ETA: oops, I just learned that that article was later retracted without explanation.] Or as another example, the implication seems to be that just breathing inside an empty elevator (which was previously occupied within the previous 30 minutes) is quite possibly an even higher-risk activity than licking the elevator buttons. Really? Really???

(And don't get me started on masks ... Masks & googles are not only not suggested in the USA, they're actively stigmatized, despite makeshift homemade masks being I think at least somewhat effective and not contributing to the ongoing supply shortage, and goggles not being in short supply at all....)

I'm posing this as a question because I don't have enough confidence, without doing more research, to declare that our public health messaging has been so wildly misdirected (at least, the messaging I've received). Does anyone have better evidence? Or what's your take?

(Update: After reading this article and this article, I'm somewhat less concerned about air that was exhaled more than a few seconds ago, and now thinking that the main things to think about are (1) being near someone coughing, (2) being near someone talking, (3) touching surfaces then touching your face, (4) aerosols from a bathroom (apparently a SARS patient with diarrhea was a super-spreader, infecting people up to 200 meters away!). I'm very uncertain about the relative importance of these four things. I'm also not sure what "near" means, see discussion here.)


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Your Update suggests you've realized this by now, but your confusion seems to be stemming from not understanding the difference between _droplets_ (large particles, fall to ground within seconds) and _aerosols_ (small particles, can float for 10s of minutes). The reason why there is an emphasis on staying 6 feet away and being careful of touching contaminated surfaces, but not an emphasis on ventilation, is because it is thought that SARS-CoV2 is mostly expelled as droplets and not as aerosols. The purported contraction disappears in this light.

Droplets are larger particles that fall to the ground within seconds, but can be expelled up to ~6 ft / 2 meters by coughing and sneezing. Droplets can also be expelled by talking. Droplets containing the virus can directly land in the face of another person, hence the recommendations to stay 6 ft away. They can also land on surfaces, whence these surfaces become "fomites" that can pass the virus to other people via touching.

Aerosols are smaller particles that can remain in the air for longer periods and potentially be moved around by ventilation systems. I think aerosolized virus can in principle be expelled by an infected person but based on the reporting and scientist interviews I've heard (admittedly mostly on This Week in Virology), my understanding is that experts think that SARS-CoV2 is mostly being expelled and hence spread by droplets and not aerosols. Aerosolization might be more important in some situations, such as by certain types of high energy toilets acting on fecally shed virus, but perhaps more important when patients are intubated in hospitals to go on ventilators. And aerosolized virus might turn out to be more common and play a more important role in SARS-CoV2 transmission than experts currently think and so it's perhaps still something to be aware of as an individual and no doubt warrants more research.

Regarding how long aerosols remain in the air...I am not familiar with the retracted article you mentioned, but the NYT reported that the authors of the now famous aerosol + surface stability study said that aerosols of SARS-CoV2 stayed in the air for 1/2 an hour. The paper itself doesn't contain this 1/2 hour number, and the authors needed to use a rotating drum to keep the virus floating for 3 hours. My understanding is that the 1/2 hour floating time has nothing to do with SARS-CoV2 itself and is just a property of physics and the size of the particles. Which is still a long time. But the question is whether significant aerosolized virus is being produced by infected people in normal circumstances. In the above paper, the authors used a nebulizer to aerosolize live virus, it didn't happen naturally.

Note: I have not reviewed the scientific evidence that the CDC and other experts have used to draw the conclusion that droplets and contaminated surfaces are more important than aerosols for SARS-CoV2 transmission.

Thanks! Yes, I think we're mostly on the same page, except my feeling that avoiding inhaled droplets is probably more important than avoiding touching your face. I still think that the public-health messaging I've seen is somewhat misdirected, and that all the celebrities and informational signs that say "wash your hands" or "don't shake hands" should instead more often say "don't have face-to-face conversations" and "if you talk to someone, you're spitting on them" and "cover your cough". The 2 meter rule is good, but I think it hasn't been communicated w

... (read more)

Just a note that there still seems to be disagreement and a lot of uncertainty about aerosol transmission https://www.nature.com/articles/d41586-020-00974-w

Where I live (USA, Kentucky), we've been under something much like "shelter in place" without the scary name and been engaged in social distancing, event cancellation, bars and restaurants closed, non-life sustaining businesses closed, strongly encouraged work from home, etc., etc. for over a week now. Masks & googles are being strongly encouraged in Kentucky to be crafted, sourced and donated for front-line healthcare and emergency workers plus at-risk populations. We're hearing massive messaging about the grave risks of being indoors in a public, poorly-ventilated space. Critical retailers like groceries and pharmacies are closing to sanitize then offering their first hour of being re-opened to at-risk populations, mostly the elderly, to reduce airborne transmission.

Are we in the same USA?

I'm very happy to hear that! Here in Massachusetts, we are doing many of the same things. It's just that that list of things somehow never coalesced into a message in my mind: "Don't breathe air that has been recently exhaled by someone else!" as a general principle for guiding my decisions and actions. I have heard "Wash your hands" 5000 times from officials, bosses, celebrities, etc., but I have never once heard that.

Are we overemphasizing touching rather than breathing?

I suspect that governments are favoring actionable advice over accuracy.

The west in general ran out of masks due to government incompetence and complacency, therefore they told us that masks don't work, because telling people that masks work but oops we don't have any would contribute to panic and discontent.

The virus spreads mostly through the air, but people can't easily stop that, so they emphasize hand-washing because it is actionable and gives people something to do and thereby reduces their anxiety. It is probably also slightly helpful - people already don't wash their hands enough and a bit more hand-washing has a low cost.

It's happened before. In WWII, they told people to collect various materials for the war effort, at least some of which were totally useless.

But we could supply the entire country with fabric masks in like a week, even assuming there's not already enough of them sitting around. People who sew live for this kind of thing.

Yeah but someone in the government would have to organise that, and they would take the blame if even one tiny thing went wrong. Telling people to wash their hands is very safe and requires no effort. It is also helpful advice because some people actually have poor hand hygiene.
This seems to not only be false in general, but definitely false in this case. I've read about a lot of people making masks. Are you claiming that only governments could supply everyone with face masks ("in a week")? I'm more in agreement with that, but I think governments could spur private parties to do it themselves just by making the suggestion too.
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I am in no way saying this relates to COVID19 but to give an example of a virus that can spread a long distance 'in the wind'.

"foot and mouth disease" caused by a picornavirus affects cattle, pigs, sheep, goats (+ other cloven-footed animals).

... Virus was reported to travel over water >250 km (155 miles) from Brittany, France, to the Isle of Wight, UK, in 1981, but it usually travels no more than 10 km (~6 miles) over land. SOURCE